runtime: c-shared entrypoint for linux/arm64

Change-Id: I7dab124842f5209097a8d5a802fcbdde650654fa
Reviewed-on: https://go-review.googlesource.com/16395Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/rt0_linux_arm64.s

@@ -9,6 +9,50 @@ TEXT _rt0_arm64_linux(SB),NOSPLIT,$-8
 	ADD	$8, RSP, R1	// argv
 	BL	main(SB)
 
+// When building with -buildmode=c-shared, this symbol is called when the shared
+// library is loaded.
+TEXT _rt0_arm64_linux_lib(SB),NOSPLIT,$40
+	// R27 is REGTMP, reserved for liblink. It is used below to
+	// move R0/R1 into globals. However in the standard ARM64 calling
+	// convention, it is a callee-saved register.
+	MOVD	R27, 24(RSP)
+
+	MOVD	R0, _rt0_arm64_linux_lib_argc<>(SB)
+	MOVD	R1, _rt0_arm64_linux_lib_argv<>(SB)
+
+	// Create a new thread to do the runtime initialization and return.
+	MOVD	_cgo_sys_thread_create(SB), R4
+	CMP	$0, R4
+	BEQ	nocgo
+	MOVD	$_rt0_arm64_linux_lib_go(SB), R0
+	MOVD	$0, R1
+	BL	(R4)
+	B	restore
+
+nocgo:
+	MOVD	$0x800000, R0                     // stacksize = 8192KB
+	MOVD	$_rt0_arm64_linux_lib_go(SB), R1
+	MOVD	R0, 8(RSP)
+	MOVD	R1, 16(RSP)
+	MOVD	$runtime·newosproc0(SB),R4
+	BL	(R4)
+
+restore:
+	MOVD	24(RSP), R27
+	RET
+
+TEXT _rt0_arm64_linux_lib_go(SB),NOSPLIT,$0
+	MOVD	_rt0_arm64_linux_lib_argc<>(SB), R0
+	MOVD	_rt0_arm64_linux_lib_argv<>(SB), R1
+	MOVD	$runtime·rt0_go(SB),R4
+	B       (R4)
+
+DATA _rt0_arm64_linux_lib_argc<>(SB)/8, $0
+GLOBL _rt0_arm64_linux_lib_argc<>(SB),NOPTR, $8
+DATA _rt0_arm64_linux_lib_argv<>(SB)/8, $0
+GLOBL _rt0_arm64_linux_lib_argv<>(SB),NOPTR, $8
+
+
 TEXT main(SB),NOSPLIT,$-8
 	MOVD	$runtime·rt0_go(SB), R2
 	BL	(R2)